Although there are various methods of taking drugs, injection is favoured by some users as the full effects of the drug are experienced very quickly, typically in five to ten seconds. It also bypasses first-pass metabolism in the liver, resulting in higher bioavailability and efficiency for many drugs (such as morphine or diacetylmorphine/heroin; roughly two-thirds of which is destroyed in the liver when consumed orally) than oral ingestion would, meaning users get a stronger (yet shorter-acting) effect from the same amount of the drug. This shorter, more intense high can lead to a dependency—both physical and psychological—developing more quickly than with other methods of taking drugs. As of 2004[update], there were 13.2 million people worldwide who used injection drugs, of which 22% are from developed countries.[1]

There are a variety of reasons why drugs would be more attractive to inject rather than take through other methods, such as:

Increased effect — Injecting a drug intravenously means that more of the drug will reach the brain more quickly. This means that the drug will have a very strong and rapid onset. With some drugs, this can produce sensations not found with other routes of administration, known as a rush.

More efficient usage — A smaller amount is enough as injection means that more of the drug will reach the brain than with other methods. This is because the body's defenses and detoxifying mechanisms (such as first-pass metabolism in the liver with oral use) are bypassed. Injection increases a drug's bioavailability. This means that it requires less drug (and thus less money) to achieve the same effect (ignoring the effects of tolerance).

In addition to general problems associated with any IV drug administration (see risks of IV therapy), there are some specific problems associated with the injection of drugs by non-professionals, such as:

Increased chance of overdose — Because IV injection delivers a dose of drug straight into the bloodstream, it is harder to gauge how much to use (as opposed to smoking or snorting, where the dose can be increased relatively incrementally until the desired effect is achieved; this gives a user who is in danger of overdosing a chance to seek medical treatment before respiratory arrest sets in). In addition, because of the rapid onset of intravenous drugs, overdose can occur very quickly, requiring immediate action.

Scarring of the peripheral veins — This arises from the use of blunt injecting equipment. This is particularly common with users who have been injecting while in jail and re-use disposable syringes sometimes hundreds of times. IV drug use for an extended period may result in collapsed veins. Though rotating sites and allowing time to heal before reuse may decrease the likelihood of this occurring, collapse of peripheral veins may still occur with prolonged IV drug use. IV drug users are among the most difficult patient populations to obtain blood-specimens from because of peripheral venous scarring. The darkening of the veins due to scarring and toxin buildup produce tracks along the length of the veins and are known as track marks.

Increased chance of addiction — The heightened effect of administering drugs intravenously can make the chances of addiction more likely.[citation needed]

Social stigma — In many societies, there is a social stigma attached to IV drug use, in addition to the more general stigma around illegal drug use and addiction. People who are satisfied taking drugs by other routes may not inject. This may be because of its perceived prevalence in inner cities and with lower-income people.

A clandestine kit containing materials to inject drugs, a bottle of a type of lean, promethazine, an antiemetic, and unidentified pills.

The drug—usually (but not always) in a powder or crystal form—is dissolved in water, normally in a spoon, tin, bottle cap, the bottom of a soda can, or another metal container. Cylindrical metal containers—sometimes called "cookers"—are provided by needle exchange programs. Users draw the required amount of water into a syringe and squirt this over the drugs. The solution is then mixed and heated from below if necessary. Heating is used mainly with heroin, (though not always, depending on the type of heroin),[3] but is also often used with other drugs, especially crushed tablets. Cocaine HCl (powdered cocaine) dissolves quite easily without heat. Heroin prepared for the European market is insoluble in water and usually requires the addition of an acid such as citric acid or ascorbic acid (Vitamin C) powder to dissolve the drug. Due to the dangers from using lemon juice or vinegar to acidify the solution, packets of citric acid and Vitamin C powder are available at needle exchanges in Europe. In the U.S., vinegar and lemon juice are used to shoot crack cocaine. The acids convert the water-insoluble cocaine base in crack to a cocaine salt (cocaine acetate or cocaine citrate here), which is water-soluble (like cocaine hydrochloride).

Once the drugs are dissolved, a small syringe (usually 0.5 or 1 cc) is used to draw the solution through a filter, usually cotton from a cigarette filter or cotton swab (cotton bud). "Tuberculin" syringes and types of syringes used to inject insulin are commonly used. Commonly used syringes usually have a built-in 28 gauge (or thereabouts) needle typically 1/2 or 5/8 inches long.

The preferred injection site is the crook of the elbow (i.e., the Median cubital vein), on the user's non-writing hand. Other users opt to use the Basilic vein; while it may be easier to "hit", caution must be exercised as two nerves run parallel to the vein, increasing the chance of nerve damage, as well as the chance of an arterial "nick".[4]

A sterile and safe legitimate injection kit obtained from a needle exchange program

Harm reduction is an approach to public health intended to be a progressive alternative to an approach requiring complete abstinence from drug use. While it does not condone the taking of illicit drugs, it does seek to reduce the harms arising from their use, both for the person taking illicit drugs and the wider community.

A prominent method for addressing the issue of disease transmission among intravenous drug users are needle exchange programs, in which facilities are available to exchange used injection equipment for safe sterile equipment, often without a prescription or fee. Such establishments also tend to offer free condoms to promote safe sex and reduce disease transmission. The idea is to slow disease transmission and promote public health by reducing the practice of sharing used needles. In countries where harm reduction programs are limited or non-existent, it is quite common for an IV user to use a single needle repeatedly or share with other users. It is also quite uncommon for a sterilizing agent to be used.

A philosophy of harm reduction promotes information and resources for IV drug users. General guidelines on safer injecting of various substances intravenously are typically based on the following steps:

The area for drug preparation should be cleaned with warm soapy water or an alcohol swab to minimize the risk of bacterial infection.[5]

The equipment required involves new syringes and needles, alcoholic swabs, rinse eye drops as sterile water, filter (cotton or syringe filter like Sterifilt), tourniquet, and a clean spoon or Stericup. In order to minimize the chance of bacteria or viruses entering the bloodstream, people are advised to wash their hands with soap and warm water. However, as people do not always have access to hot water and soap when they are injecting, the philosophy of harm reduction seeks to find the most realistic and reliable option that drug users will take; a process that takes much time or access to material is unlikely to be used frequently. Alcohol swabs are commonly distributed with injecting equipment, and while they are less effective than hand washing, their use is more effective than nothing. Any sharing of injecting equipment, even tourniquets, is highly discouraged, due to the high danger of transmitting bacteria and viruses via the equipment.[5]

Sterile water is also recommended to prevent infection. Many needle and syringe programs distribute vials or ampoules of USP sterile water for this reason. Where sterile water is not obtainable, the harm reduction approach recommends tap water boiled for five minutes, and then allowed to cool.[5]

Once the water and substance are combined in the mixing vessel, heat is sometimes applied to assist the mixing. Filtering is recommended by health services, as the mix can consist of wax or other non-soluble materials which are damaging to veins. Additionally, the injection of talc has been associated with pulmonary talcosis in intravenous drug users.[6]Wheel filters are the most effective filters.[7] 5.0 micron wheel filter (e.g., Apothicom Sterifilt)—now shared in some needle exchange programs instead of cotton—is intended to get rid of the talc from prescription tablets like benzodiazepines, dextroamphetamine, methadone tablets, and other recreational drugs like MDMA. However, cotton wool (with the risk of cotton fever) or tampons can be used, although to be more effective, several filtrations should be performed; cigarette filters should not be used, due to the risk of fibres breaking off and being injected along with the solution, nor should filters of any sort ever be re-used, either as filters or in an attempt to recover drug material present, due to many risks, ranging from cotton fever to life-threatening sepsis.[5]

Once the mix is drawn into the syringe, air bubbles should be removed by flicking the barrel with the needle pointed upwards and pressing the plunger to expel the bubbles that pool at the top. This is done to prevent injection of air into the bloodstream.[5] The potential danger of an air embolism is often greatly overestimated by IV users; up to three CCs of air can be injected intravenously without causing complications, and time spent meticulously getting every minute air bubble out of the syringe would often be better spent ensuring clean conditions in general.

A tourniquet can be used to assist vein access. The tourniquet should not be on too tight, or left on for too long, as this causes the veins to swell and stretch. When injecting, the needle's bevel or "hole" should face upward and be eased into the vein at a shallow angle between 10 and 35 degrees to minimize the risk of penetrating through the vein entirely. In order to prevent stress on the vein, the needle should be pointing towards the heart.[5]

The plunger should be pulled back slightly (colloquially known as "jacking back" or "flagging") to ensure the needle is in the vein. Blood should appear in the barrel of the syringe if this is the case. This process is termed aspirating the needle or registering. When accessing a vein with unobstructed blood flow, a "flashback"—or sudden flash of red blood inside the needle tip—may occur spontaneously when the needle enters the vein. Because sudden appearance of blood in the needle/syringe alone does not guarantee proper needle placement (flashbacks can also occur when a needle passes through a vein completely, enters an artery inadvertently, or otherwise is extravasated), aspirating the plunger on the syringe is still considered a requisite step.[5]

The tourniquet should then be taken off and the plunger gently pushed. After injection, a clean tissue or cotton wool should be pressed against the injection site to prevent bleeding. Although many people use an alcohol swab for this purpose, it is discouraged by health services as the alcohol interferes with blood clotting.[5]

Dispose of injecting gear using a "sharps bin" if supplied. Other rigid-walled containers such as a bottle are recommended as a second best option.[5]

An estimated 16 million people worldwide use intravenous drugs, and approximately 3 million of these are believed to be HIV positive.[8] The main symptoms for any blood-borne infections will usually appear a few days after infection has occurred and usually consists of a blocked and/or runny nose; loss of taste, smell, and/or other senses; and an unpleasant sense of thickness in the forehead. A general feeling of malaise, aching, and weakness will usually accompany these symptoms. If the onset of symptoms happens around 4–8 days after infection, then it is likely hepatitis, but could also be any strain of HIV. Sufferers tend to get these same symptoms regardless of what disease or virus they may have contracted.

The most common symptoms of HIV or AIDS that has been contracted intravenously are again a runny and/or blocked nose, acute loss of taste and/or smell, a blocked or thick sensation within the head, general aching, malaise and weakness, hot and cold sweats, and occasionally acute insomnia. These symptoms will most likely subside after 2–3 days, and the individual will then regain their previous posture and well being. An individual could possibly live completely unaware of the presence of the virus for many years as the initial symptoms subside and may not appear again for a long time.[9]

Of all the ways to ingest drugs, injection carries the most risks by far as it bypasses the body's natural filtering mechanisms against viruses, bacteria, and foreign objects. There will always be much less risk of overdose, disease, infections, and health problems with alternatives to injecting, such as smoking, insufflation (snorting or nasal ingestion), or swallowing.

Viruses such as HIV and hepatitis C are prevalent among IV drug users in many countries, mostly due to small groups sharing injection equipment combined with a lack of proper sterilization. Other health problems arise from poor hygiene and injection technique (be it IV, IM, or SC), such as cotton fever, endocarditis, phlebitis, abscesses, vein collapse, ulcers, malaria, gas gangrene, tetanus, septicaemia, thrombosis, embolism, and all results thereof. Drug injection is also commonly a component in HIV-related syndemics. Fragments from injection of pills are known to clog the small blood vessels of the lungs, brain, and elsewhere, potentially causing pulmonary embolism (PE), stroke, or venous embolism. A small proportion of PE is due to the embolization of air, fat, and talc in the drugs of intravenous drug abusers. More commonly, the inflammatory response to these foreign objects causes granulation tissue to form in the capillary beds, resulting in vasculitis, and, when it occurs in the pulmonary capillary bed, potentially pulmonary talcosis. Hitting arteries and nerves is dangerous, painful, and presents its own similar spectrum of problems.

Particularly for intravenous administration, self-injection in the arm can be awkward, and some people modify a syringe for single-handed operation by removing the plunger and affixing a bulb such as from a large dropper or baby pacifier to the end of the barrel to in effect make it a large dropper with a needle affixed. This is therefore a variant of the common method of injection with a dropper with the hypodermic needle affixed, using a "collar" made of paper or other material to create a seal between the needle and dropper. Removing part of the plunger assembly by cutting off most of the shaft and thumb rest and affixing the bulb to the end of the barrel, thereby allowing the bulb to operate the plunger by suction, also does work in many cases.

An alternative to syringes in the 1970s was to use a glass medicine dropper, supposedly easier to manipulate with one hand.[10] A large hairpin was used to make a hole in the skin and the dropper containing the drug (usually heroin) was inserted and the bulb squeezed, releasing it into the tissues.[11] This method was also reported—by William S Burroughs and other sources—for intravenous administration at least as far back as 1930.

Insufflation (snorting or sniffing) is usually safer than injection in terms of the relative danger of transmission of blood-borne viruses. However, the membranes in the nose are very delicate and can rupture when snorting, so users should have their own snorting equipment not shared with anyone else, in order to prevent viral transmission. As with injection, a clean preparation surface is required to prepare a drug for snorting. Nasal membranes can be seriously damaged from regular snorting.

Drugs can also be smoked or "chased". Smoking and chasing have negligible risk of bacterial or viral transmission and the risk of overdose is lessened compared to injecting, but they still retain much of the "rush" of injecting as the effects of the drug occur very rapidly. Chasing is a far safer way to use heroin than injecting, with one common option being to use new aluminum foil, first passing a cigarette lighter flame over both sides/or just the shiny side at least, which is to help sterilize it ("curing").[citation needed]

Swallowing tends to be the safest and slowest method of ingesting drugs. It is safer as the body has a much greater chance to filter out impurities. As the drug comes on slower, the effect tends to last longer as well, making it a favorite technique on the dance scene for speed and ecstasy. People rarely take heroin orally, as it is converted to morphine in the stomach and its potency is reduced by more than 65% in the process. However, oral bioavailability of opioids is heavily dependent on the substance, dose, and patient in ways that are not yet understood.[12] Pills like benzodiazepines are best swallowed as they have talc or wax fillers in them. These fillers won't irritate the stomach, but pose serious health risk for veins or nasal membranes.

Administering oral tablets sublingually (under the tongue) or bucally (between the gum and jaw) is a technique used clinically to increase bioavailability of many drugs. A notable increase in the effects of a drug taken this way is noted for many of the opioids and nearly all of the benzodiazepines.

"Shebanging" involves spraying the dissolved drug into the nose to be absorbed by the nasal membrane.

"Plugging", or rectal ingestion, relies on the many veins in the anal passage passing the drug into the blood stream quite rapidly. Some users find that trading off some of the "rush" for fewer health risks is a good compromise. Shafting usually involves about 1.5 ml of fluid mixed with the drug.

Women have the added option of "shelving", where drugs can be inserted in the vagina. This is similar to the rectum, in that there are many blood vessels behind a very thin wall of cells, so the drug passes into the bloodstream very quickly. Care should be taken with drugs such as amphetamine that may irritate the sensitive lining of the rectum and vagina.

Substances below a certain molecular weight can be absorbed through the skin and into the bloodstream when dissolved in the solvent dimethyl sulfoxide (DMSO) which is available as liquid or gel; there therefore exists the possibility of creating a topical concoction with medical-grade DMSO and a given drug which will solve the first pass and GI tract destruction problems in addition to faster onset of effects.

IV drug use is a relatively recent phenomenon arising from the invention of re-usable syringes and the synthesis of chemically pure morphine and cocaine.

It was noted that administering drugs intravenously strengthened their effect, and—since such drugs as heroin and cocaine were already being used to treat a wide variety of ailments—many patients were given injections of "hard" drugs for such ailments as alcoholism and depression.

The hypodermic needle and syringe in its current form was invented by the French scientist Charles Pravaz in 1851, and became especially known during the wars of that and the subsequent decade. However, the first well-known attempt to inject drugs into the body was a 1667 attempt to inject a solution of opium into a dog, and some had suspected that parenteral administration of drugs may work better based on the practise of rubbing opium and other drugs into sores or cuts on the skin for the purpose of causing systemic absorption and the beginnings of scientific understanding of the functioning of the lungs.

During most of the 1850s, the previously-held belief that opiate dependence and addiction (often called "the opium appetite", or, when relevant, the "morphine appetite" or "codeine appetite") was due to the drug's action on the digestive system—just like any hunger or thirst—caused doctors to opt to inject morphine rather than administer it orally, in the hope that addiction would not develop. Certainly, by c. 1870 or earlier, it was manifest that this was not the case and the title of earliest morphine addict as the term is currently understood is often given to Pravaz' wife, although habituation through orally ingesting the drug was known before this time, including Friedrich Sertürner and his associates, followers, wife, and dog. To some extent, it was also believed early on that bypassing the lungs would prevent opium addiction, as well as habituation to tobacco. Ethanol in its usual form generally is not injected and can be very damaging by most routes of injection; in modern times, it is used as an alternative or potentiator of phenol (carbolic acid) in procedures to ablate damaged nerves.

In or shortly after 1851, the drugs which had been discovered and extracted from their plants of origin and refined into pure crystalline salts soluble in water included morphine (1804 or late 1803), codeine (1832), narcotine/noscapine (1803-1805?), papaverine (1814), cocaine (1855), caffeine (1819), quinine (1820), atropine (1831), scopolamine (aka hyoscine, aka laevo-duboisine) (1833?), hyoscyamine or laevo-atropine (1831), opium salts mixtures (c. 1840s), chloral derivatives (1831 et seq.), ephedrine (1836?), nicotine (1828), and many others of all types, psychoactive and not. Morphine in particular was used much more widely after the invention of the hypodermic syringe, and the practise of local anaesthesia by infiltration was another step forward in medicine resulting from the hypodermic needle, discovered at around the same time that it was determined that cocaine produced useful numbing of the mucous membranes and eye.

A wide variety of drugs are injected. Among the most popular in many countries are morphine, heroin, cocaine, amphetamine, and methamphetamine. Prescription drugs—including tablets, capsules, and even liquids and suppositories—are also occasionally injected. This applies particularly to prescription opioids, since some opioid addicts already inject heroin. Injecting preparations which were not intended for this purpose is particularly dangerous because of the presence of excipients (fillers), which can cause blood clots. Injecting codeine into the bloodstream directly is dangerous because it causes a rapid histamine release, which can lead to potentially fatal anaphylaxis and pulmonary edema. Dihydrocodeine, hydrocodone, nicocodeine, and other codeine-based products carry similar risks. Codeine may instead be injected by the intramuscular or subcutaneous route. The effect will not be instant, but the dangerous and unpleasant massive histamine release from the intravenous injection of codeine is avoided. To minimize the amount of undissolved material in fluids prepared for injection, a filter of cotton or synthetic fiber is typically used, such as a cotton-swab tip or a small piece of cigarette filter.

Some manufacturers add the narcotic antagonistnaloxone or the anticholinergics atropine and homatropine (in lower than therapeutic doses) to their pills to prevent injection. Unlike naloxone, atropine does indeed help morphine and other narcotics combat neuralgia. The atropine may very well not present a problem, and there is the possibility of atropine content reduction of soluble tablets by placing them on an ink blotter with a drop of water on top, then preparing a shot from the remainder of the pill. Canada and many other countries prohibit manufacturers from including secondary active ingredients for the above reason; their Talwin PX does not contain naloxone. However, as a narcotic agonist–antagonist, pentazocine and its relatives can cause withdrawal in those physically dependent upon narcotics.